Stents, grafts, stent-grafts, vena cava filters, expandable frameworks and similar implantable medical devices are radially expandable endoprostheses, which are typically capable of being implanted transluminally and enlarged radially after being introduced into a body lumen.
Stents, for example, may be implanted in a variety of bodily lumens, including blood vessels within the vascular system (e.g., coronary vessels, secondary vessels, etc.), lumens of the urinary tract, bile ducts, fallopian tubes, and so forth. Stents are commonly either balloon-expandable or self-expanding, depending upon how deployment is carried out. Balloon-expandable stents are manufactured in a crimped state and expanded to vessel diameter by inflating a balloon and plastically deforming the stent. Self-expanding stents are manufactured at or somewhat above vessel diameter and are crimped and constrained to the smaller diameter until the intended delivery site is reached, where the constraint is removed and the stent deployed. Thus, self-expanding stents, instead of being plastically deformed to the vessel diameter, expand by simply reverting to an equilibrium shape.
Stents are commonly manufactured from either metal or polymer tubes, often by laser or chemical or mechanical machining. The stent mechanical properties are typically dependent upon the properties of the material from which they are formed. Stents made of metal typically have relatively high strength, stiffness, and radiopacity and less elastic recoil upon expansion relative to stents made of polymer. This is because metals tend to have a higher Young's modulus of elasticity, higher yield strength, higher work hardening rate, and higher density than polymers. Polymer stents typically have more axial and radial flexibility than metal stents with the same wall thickness due to the polymer's lower modulus of elasticity.
The mechanical properties of polymer stents, however, typically require significant compromises in design in order to close the gap in mechanical properties relative to metal stents. For example, in order to reach the radial strength and stiffness of metal stents, polymer stents commonly need to have a wall thickness that is significantly greater than the wall thickness of a comparable metal stent. This undesirably increases the profile of the polymer stent such that it occupies more of the vessel luminal area, thus reducing the volume of fluid flow in the stented lumen.